Electrolyzers of this type are known. For example, one electrolyzer, adapted to effect the electrolysis of water, comprises at least one porous cathode, and at least one porous anode which can be of a noble metal, for example platinum, these electrodes being immersed in a suitable aqueous bath of electrolyte such as a dilute solution of a strong mineral acid. At the time of operation of this electrolyzer, hydrogen is released at the interface between the electrolyte and the cathode, that is to say, at the surface of the cathode in contact with the electrolyte, in the form of gas bubbles. Additionally, and simultaneously, oxygen is released in the form of bubbles at the interface between the electrolyte and the anode.
Another known electrolyzer, adapted for the production of chlorine and the manufacture of sodium, by electrolysis of an aqueous solution of sodium chloride, comprises at least one porous cathode of a metal such as nickel or iron, and at least one porous anode which can be a metal such as titanium, or graphite. These electrodes are immersed in the electrolyte and anodic and cathodic compartments are separated by a porous diaphragm which can be of asbestos or porcelain or a suitable metal. At the time of operation of this electrolyzer, there is produced a release of bubbles of hydrogen at the cathode-electrolyte interface and bubbles of chlorine at the anode-electrolyte interface.
The gas release discussed hereinabove, is produced in major portion at the active surfaces of the electrode which are facing the electrodes of opposite polarity, that is to say along lines of electrolysis current. There results therefrom an interruption of a portion of the latter for each release of gas bubbles which produces a polarization of the electrodes and diminishes the electrochemical yield of the electrolysis.
Swiss Pat. no. 480,870 describes an electrode with gas diffusion adapted to be utilized in an electrolyzer of the same type as that which is the object of the present invention.
This electrode is composed of a porous layer of electrochemically active material (designated a "work layer"), a layer of porous material electrochemically inactive serving the function of a mechanical support for the electrode and an intermediate layer of electrically insulative, porous material, interposed between the "work layer" and the "support layer", the mean value of the pore size of this intermediate layer being less than that of the pore size of the "work layer".
Although this electrode avoids the release of gas in the electrolyte just discussed, it presents nevertheless the disadvantage of being the source of a substantial voltage drop, being able to reach 10% of the theoretical voltage of the electrolyzer, which is caused by the presence of the support layer.
In addition, in the case where the "support layer" is of an electrically conductive material, for example, a metal, the current density in the electrolyzer is limited to a value such that the resistance drop which appears between the two faces of the "support layer" would be less than the electrolysis voltage of the electrolyte (for example, in the case where this latter is an aqueous electrolyte, this resistance drop must not be less than about 1.23 volts). In fact, for a resistance drop at least equal to the electrolysis voltage of the electrolyte, the conductive support layer would be composed at least partially of a bipolar electrode and would be the source of undesirable parasitic release of gas.
Additionally, the utilization of an electrically conductive "support layer" introduces the risk of a short circuit with the active electrode which requires the electrical isolation of these two members in order to avoid the gaseous release from the "support layer". For analogous reasons, a metallic "support layer" must be electrically insulated from the counter electrode of the electrolyzer which it faces. This makes necessary the disposition between the two electrodes, facing one another in the electrolyzer, of electrical insulation means such as a separator. The presence of such separator leads to a resistance drop and additonal encumbrance.
Finally the volume occupied by the "support layer" increases the total volume of the electrolyzer without increase of its power.